The Effect of Diode Laser Irradiation on Surface Roughness of Bleached Enamel : An in Vitro Study

Aims: To evaluate the influence of post bleaching diode laser irradiation on the surface roughness of bleached enamel. Materials and methods: Forty bovine teeth were prepared and then randomly divided into two equal groups (n=20) according to the in–office bleaching technique as follows: CB: Conventional in–office bleaching technique LB: Laser–assisted in–office bleaching technique. Each group was then subdivided into two subgroups (n=10) according to laser irradiation setting as follows: C2: Conventionally bleached followed by 2-Watt laser irradiation, C4: Conventionally bleached followed by 4-Watt laser irradiation, L2: Laser–assisted bleached followed by 2Watt laser irradiation and L4: Laser–assisted bleached followed by 4Watt laser irradiation. Surface roughness was assessed three times for all the specimens: before bleaching, 24 hours after bleaching and finally 24 hours after post bleaching diode laser irradiation by utilizing profilometer. Paired samples t–test and independent samples t– test was utilized for statistical analysis at p≤0.05. Results: Both bleached groups exhibited a significant increase in the surface roughness of enamel with no statistically significant difference between them. Post bleaching diode laser irradiation at 4Watt resulted in a significant increase in surface roughness of enamel, while 2Watt diode laser irradiation resulted in non–significant increase in surface roughness of enamel. Conclusions: In–office bleaching techniques had a negative impact on enamel in terms of surface roughness. Post bleaching diode laser irradiation at 2Watt power parameter is more preferred than 4Watt power parameter.


INTRODUCTION
Growth in patient's awareness about esthetics caused an increase in the popularity of dental bleaching procedure. It can be classified generally into two basic types: In-office dental bleaching procedure, which is accomplished by the dental professional at dental clinics and at-home dental bleaching procedure in which the patient himself applies the bleaching material into custom made tray that fits his teeth at home and under the supervision of the dental professionals (1) . The in-office dental bleaching technique is carried out by using higher concentrations of bleaching agent (35%-38 % carbamide peroxide or 25%-40 % hydrogen peroxide) for short duration of bleaching time, while lower concentrations of bleaching agent (10%-16 % carbamide peroxide or 3%-6 % hydrogen peroxide) are utilized in athome bleaching technique for long duration (1) .
The precise mechanism of tooth bleaching is not completely understood, but it can be attributed to the changes that may occur in the chemical structures of tooth by free radicals that are released as a result of hydrogen peroxide decomposition by oxidation reduction reactions (2) . These free radicals are considered as extremely unstable agents because they have in their outer orbital one or more unpaired electron (3) . They have great tendency to acquire electron from adjacent compounds for stabilization of their molecular structure. As consequence, the pigmented molecules within the dental structure become oxidized leading to decrease or elimination of tooth discoloration (3) .
A relatively new advancement in the field of in-office dental bleaching technique is the activation of bleaching agent by laser light by incorporating specific chromophores in bleaching material that are capable of absorbing specific wavelength of laser (4) . Although the efficacy of dental bleaching in improving the color of teeth is not questioned (5)(6)(7)(8) , there is a controversy with no general agreement about the safety of dental bleaching on surface roughness of enamel (7,9) . A wide range of lasers has been utilized as post bleaching treatment modalities in an attempt to enhance the microhardness of bleached enamel, which could be explained by some chemical and physical alterations in enamel structures caused by photochemical or photothermal interactions induced by laser (10) . Different effects were found depending on the temperature degree reached during laser irradiation treatment (11) . The evidence in literature is very rare regarding the application of diode laser irradiation on bleached enamel and to the best of our knowledge there are limited studies investigated its effect on of enamel (12) with no previous study assessed its effect on surface roughness of bleached enamel. Thus, the aim beyond the current study was to assess the effect of in-office bleaching techniques on surface roughness of enamel and the possible influence of post bleach-parameters on surface roughness of bleached enamel by two different in-office bleaching techniques. The first null hypothesis to be tested was that in-office dental bleaching techniques had no effect on surface roughness of bleached enamel.
While the second null hypothesis was that post bleaching diode laser irradiation at two different power parameters had no effect on the surface roughness of enamel.

Specimen's collection and preparation
Forty permanent bovine incisors (13) were utilized in the present study. They were extracted by one of the investigators on the same day of cattle slaughtering to prevent the need for refrigeration and to avoid dehydration of teeth at the same time then they were stored in 0.1% thymol solution at room temperature for disinfection for one month before starting the study (14) . The remnant of soft tissues from the teeth was gently removed by using dental hand scaler. They were examined under stereo microscope (X40) to ensure that the specimens were free from caries, surface cracks or any enamel defect (15) . remove all the impurities for 15 minutes (7) . The center of the labial surface was chosen to conduct the study (13) . The target area was defined by using masking tape with 5 mm diameter hole to standardize the area of measurements. The samples were stored in deionized water during conducting the study.

Study design and specimens grouping
Forty bovine teeth were randomly al-

Statistical analysis
The normal distribution of data was verified by performing a normality test (Shapiro-Wilk). All the data showed normal distribution. Therefore, paired samples t-test and independent samples t-test were performed to compare between before and after treatment and between groups respectively. The significance level was set at 5% for all tests.

RESULTS
The data in Table (     Mean ± SD 0.149 ± 0.029 0.187 ± 0.039 0.038* L2: Laser irradiated group at 2 Watt; L4: Laser irradiated group at 4Watt; SD: Standard deviation; P: Based on paired samples t-test. *Indicates a significant difference at P ≤ 0.05. ˟ indicates non-significant difference at P ˃ 0.05.

DISCUSSION
Dental bleaching procedure is considered as efficient and relatively safe (16) . However, some harmful effect could be resulted from this procedure (17) . Bovine teeth were used in the present study because of increasing difficulty in obtaining large number of human anterior teeth for dental research with lack of the control of their age as compared to the cows that have the same nutritional condition and slaughtered within the same age range (2-3 years). Furthermore, the infection hazards and ethical issues must be considered (18) . Bovine teeth would not be subjected to any previous manufactured fluoride products that may affect the microhardness of enamel which is fundamental in this kind of researches for better standardization (19) , in addition to significantly larger flat labial surface (18) . The bovine teeth have been proposed as human teeth substitute as there was no significant difference between both in terms of microhardness and chemical composition (13) . The mean Ra value obtained after conventional bleaching technique was significantly higher than baseline values. This finding coincides with the findings of other researchers that demonstrated a significant rise in surface roughness of enamel using the same 35% concentration of H2O2 (7,20) . Dionysopoulos et al.
(2016) disagreed with our findings by reporting that 40% H2O2 alone or with Er,Cr:YSGG laser irradiation did not affect the surface roughness of enamel significantly. These contradictory findings may be explained by the fact that the specimens in the previous cited study were stored in artificial saliva for 10 days before performing surface roughness assessment (9) . Laser assisted in-office bleaching technique also resulted in a significant increase in surface roughness of enamel, which is parallel with the outcome of other study utilized the same bleaching gel (Laser white 20 whitening gel kit, Biolase, Irvine, USA) and diode laser activation device (Epic 10, Biolase, Irvine/ California, USA) (8) . In this in vitro study, no significant difference was evident between the mean of Ra value obtained after conventional and laser assisted in-office bleaching techniques. This outcome is in contrary to Anaraki et al. (2014) who found that the conventional bleaching using 40% H2O2 caused non-significant increase in the surface roughness of enamel as compared to laser-assisted bleaching utilizing diode laser that caused a significant increase in surface roughness of enamel (21) .  (24) . This thermal effect may lead to the creation of micro spaces as consequence of loss of carbonate, water and organic matrix from laser irradiated enamel (25) . The energy density of the laser can be calculated as power density multiplied by the total laser irradiation time. Thus the energy density could be increased by increasing the power parameter of diode laser (26) . As consequence, the increased energy density could cause higher thermal load of diode laser irradia-